Optical fibers, in particular laser fibers, comprise in general a glass fiber core and at least one cladding, which serves as the pump core in laser fibers. Furthermore, this construction is protected against environmental influences by a polymer coating. The glass fiber core and the cladding are made chemically in essence of silicon oxide. In order to ensure the wave guidance of the pump core, the polymer coating is made of a material having a refractive index that is less than the refractive index of silicon oxide.
In the case of the laser fibers known from the prior art, the glass fiber core is doped with at least one rare earth element and at least one additional dopant. The additional dopant(s) serves (serve) as a solubilizer for the rare earth element in the silicon oxide-based glass matrix and is (are) supposed to have a beneficial effect on the additional properties of the fiber, in particular a so-called photodarkening effect.
Optical fibers, in particular laser fibers, usually exhibit a graduated refractive index profile in their core. In this case the core can consist of multiple core layers. The core layers are fabricated, for example, by a process for chemical gas phase deposition in combination with the impregnation process. However, the high complexity of the fabrication process allows the depositions of only a few core layers. Therefore, for production related reasons graduated core refractive index profiles can be realized only with extreme effort and cost and at a very low yield and, thus, for all practical purposes are not used.
The solubility of rare earth elements in pure silicon oxide is typically limited to a few 100 mol-ppm, but can be improved with the use of solubilizers, such as aluminum or phosphorus. Therefore, solubilizers are in general customary and necessary as co-dopants in the core area of rare earth-doped laser fibers. However, the dopants that are used as solubilizers cause together with the doped rare earth element mechanical stresses at the preform core interface or more specifically at the fiber core interface. Such stresses at higher solubilizer concentrations or more specifically rare earth element concentrations result in undesired glass defects during the fabrication and processing of the optical fiber. Predominantly mechanical stresses occur at the interface between the core and the cladding and can result in the destruction of the preform or more specifically the fiber.
Furthermore, optical fibers, which are provided as the laser fibers for the high power range; have to be configured in such a way that the undesired non-linear optical effects, such as the so-called Raman or the so-called Brillouin effect, are suppressed. In the simplest case this is done by keeping the laser fiber as short as possible, because the intensity of many non-linear effects scales with the length of the laser fiber.
In order to realize short fiber lengths of typically 10 m for a laser fiber for the high power range, the absorption in the cladding has to be as high as possible and amount to, for example, 1.5 dB/m. The required high cladding absorption can be achieved, in principle, in two ways:
1. A high core absorption of the laser fiber is targeted. This goal is achieved by means of a high rare earth concentration in the laser core.
2. The smallest possible ratio between the cladding area and the core area is realized.
In the interest of a large area for coupling the pump radiation into the pump core or more specifically the cladding, the cross sectional area of the core has to increase correspondingly. However, any effort to enlarge the core area is impeded by technological constraints. First of all, when enlarging the core cross section it must be observed that the so-called single mode operation of the core and, thus, the resulting good beam quality is maintained. Therefore, as the core area increases, the numerical aperture of the laser core has to be decreased according to the equation2n·a·NA/λ=const.where a is the radius of the core; NA is its numerical aperture; and λ is the wavelength of the light to be guided in the core. Fibers having a relatively large core diameter and a relatively small numerical aperture of the core are typically called large mode area fibers (LMA fibers). However, any effort to decrease the numerical aperture to values below approximately 0.05 is impeded by technological constraints, especially if the chemical gas phase deposition process is used in combination with the impregnation doping process.
As the core absorption increases with an increase in the rare earth concentration, the refractive index of the core also increases, because rare earth elements act as refractive index increasing dopants. In addition, rare earth compounds—in particular, ytterbium-III oxide Yb2O3 which is typically used in high power laser fibers—have an especially high coefficient of thermal expansion compared to other commonly used core dopants (F. Just, H.-R. Müller, H-Bartelt; Mechanical stresses in rare-earth doped fiber preforms, German Society of Applied Optics DGaO [Deutsche Gesellschaft für angewandte Optik, German Branch of the European Optical Society] Proceedings 2008).
For example, the coefficient of thermal expansion of Yb2O3 at a value of 4.1×10−7 (K·mol %)−1 is significantly higher than the thermal expansion coefficient of the co-dopants that are commonly used—Al203, P205, GeO2, B2O3. A high ytterbium doping in the core area generates such high mechanical stresses compared to the undoped cladding area that the preform or more specifically the fiber core can burst during the cooling phase.
The change in the thermal expansion coefficient based on the dopant concentration is shown for a variety of dopants in the following table:
Change in the thermal expansion coefficient based on the dopantDopantconcentrationYb2O3 4.1 × 10−7 1/(K · mol %)P2O5 1.5 × 10−7 1/(K · mol %)Al2O30.53 × 10−7 1/(K · mol %)SiF4−0.5 × 10−7 1/(K · mol %)
An exemplary overview of the relationship between the refractive index and the thermal expansion coefficient of a doped silicon glass is shown for a variety of co-dopants in FIG. 1 (Chin-Lin Chen, Foundations for guided-wave optics, Wiley Interscience 2007, p. 283). FIG. 2 shows exemplary thermal expansion coefficients for doped quartz glass as a function of a few co-dopants.
Furthermore, laser fibers, which are intended for use in the high power range, must be distinguished by low photodarkening losses, in order to be able to work in a stable way for a long time. In order for a laser fiber to show low photodarkening, the laser core is doped not only with the rare earth elements in general, but also with at least one additional co-dopant. In this context high Al2O3 and/or P2O5 concentrations (3 to 10 mol %) have proven to be especially good (S. Jetschke, Photodarkening in Yb doped Optical Fibers, Institute of Photonic Technology [IPHT] Presentation, Jun. 13, 2008). In order to ensure an adequately high solubility of the rare earth elements in the glass matrix, sufficiently large quantities of co-dopants, such as Al2O3 or P2O5, are also necessary.
When the laser is operating at a high laser output power (1800 W), the laser fibers in the core area reach temperatures of up to approximately 634° C. and above (D. C. Brown, H. J. Hoffmann, Thermal, Stress, and Thermo-Optic Effects in High Average Power Double-Clad Silica Fiber Lasers, IEEE Journal of Quantum Electronics, Vol. 37, 2 Feb. 2001, pp. 207-217). The resulting heat that is generated in the fiber core has to be dissipated. Therefore, high power fibers are usually cooled actively or passively from the outside, in order to dissipate the heat generated in the core area and to prevent thermal destruction of the outer polymer coating. However, the outer cooling and the high thermal load in the fiber core cause in turn high thermal stresses, which are generally intensified by the dopants and which may result in the mechanical destruction of the fiber core.
The rupturing of the fiber core is promoted in fibers, which as core glass Al2O3—P2O5—Yb2O3—SiO2 due to the high crystallization tendency of species of the type AlPO4 and/or Al(OP)4 or P(OAl)4 of the core glass, especially during prolonged cooling. Therefore, it is possible for crystallization and stress induced destruction of the fiber core to occur as early as during the fabrication of the preform or during the subsequent fiber drawing process (C. C. de Araujo, L. Zhang, H. Eckert, Sol-gel preparation of AlPO4—SiO2 glasses with high surface mesoporous structure, J. Mater. Chem. 2006, No. 16, pp. 1323-1331). The mechanical stresses in the core/cladding interface can generate crystalline nuclei that form the conditions for glass crystallization and become apparent especially with respect to the negative effect of photodarkening.